1. Field of the Invention
The present invention relates to the preparation of phenolic esters of aromatic carboxylic acids and more particularly to the preparation of diphenyl esters of aromatic carboxylic acids, such as phthalic acid, to provide ester monomer products having good color properties.
2. Description of the Prior Art
The production of various phenolic esters of aromatic benzenedicarboxylic acids, such as diphenyl terephthalate and monophenyl terephthalate, has become of significant commercial interest in recent years due to their use in a great many types of processes. As an example, diphenyl terephthalate and diphenyl isophthalate when dissolved in a solvent may be reacted with a primary diamine to produce polyamides. Likewise, 3,3'-diaminobenzidine may be condensed with various diphenyl esters to form polybenzimidazoles. In the synthesis of commercial resins, such as Durel.RTM., diphenyl phthalates are reacted with bisphenol A to provide polyarylate resins which find extensive commercial use. The diphenyl phthalate esters may be produced by reaction of acid chlorides with a phenol to produce the phenyl ester and hydrogen chloride as a by-product, or by reacting a phenolic compound and aromatic dicarboxylic acid in the presence of a catalyst consisting of an alkali metal compound and boron compound. Other methods of preparation include catalyzing the reaction of phenol and phthalic acid with strong acids, such as alkylsulfonic acid, or with organotitanate catalysts such as titanium-n-butoxide. While such processes are effective for producing phenolic esters they suffer the disadvantage that during the process of esterification, the ester acquires a pink to dark brown color which methods of purification such as vacuum distillation, recrystallization and/or carbon treatment, are insufficient to remove. With organotitanate catalysts, for example, the titanates form extremely colored complexes with phenoxy moieties which appear to contribute to the dark color of the recovered diphenyl phthalate and which are carried through when production of polyarylate resins. According, there is a need the monomer is subsequently reacted with bisphenol A in the for additional preparational methods that can produce specification grade diphenyl phthalates having good quality color properties.
U.S. Pat. No. 2,720,504 discloses preparation of polyesters in the presence of catalyts which are alkali metal and alkaline earth metal salts containing a complex zirconium hexalkoxide radical. U.S. Pat. No. 2,930,785 discloses a process for polymerizing olefins in the presence of a catalyst composition comprising a zirconium derivative, such as zirconium propionate and zirconium butoxide. U.S. Pat. No. 3,056,818 discloses a method for preparing esters in the presence of an organotitanium or organozirconium catalyst in which the organic group can be an alkoxyl group, such as butoxy titanium hexanoate by reacting aromatic acids, such as isophthalic and terephthalic acids, with alcohols such as phenylethanol. U.S. Pat. No. 3,927,052 discloses a polymerization catalyst prepared by reacting an organosilicon compound with an organozirconium compound to prepare glycol-dimethyl terephthalate polyesters. U.S. Pat. No. 4,241,216 discloses the preparation of phthalate diesters in the presence of lower alkyl zirconium esters. U.S. Pat. No. 4,444,904 relates to a process for preparing zirconium hydrocarboxide catalyst compositions such as tetraisopropoxide and tetra-n-propoxide. U.S. Pat. No. 4,440,946 discloses a reduced silver-cadmium-zinc-zirconium catalyst for producing carboxylate esters.